ANYONE who looks at an unexceptional and unattached one-family American house, and then studies an apartment with an equal number of rooms tucked in the midst of 99 others in a six-story apartment building with brick walls, will conclude that it is cheaper to heat the apartment than the one-family house. Less outside wall surface per apartment.  Fewer windows per square foot of wall. Heavier, cheaper oil in a larger, more powerful burner. Fewer square feet of roof per room.  And so on.